Read heads are commonly used to retrieve data from storage media in high-performance data storage devices. For example, magneto-resistive heads are used to read data from hard disk drives and from floppy disks in computer related applications. A magneto-resistive head reads digital information from a magnetic storage media and transfers the digital information as a differential voltage signal, also called a response signal, to a read channel that retrieves the digital information. The digital information is stored on the media as a series of magnetic flux changes. A typical response to an isolated magnetic flux change on the media is an electric pulse whose polarity depends on the direction of the magnetic flux change.
Magneto-resistive heads often produce an asymmetric response signal, or, in other words, a response signal with an asymmetry. The asymmetry is defined as the difference between the amplitude of positive and negative peaks in the asymmetric response signal normalized by their average value, which is expressed according to equation (1): ##EQU1## where peakP and peakN denote the amplitude of positive and negative isolated pulses in the asymmetric response signal. Typical values for the asymmetry range from 10% to 20% even though higher values (up to 30%) can be measured.
An asymmetric response signal produced by a magneto-resistive head may be represented mathematically by a function g(t). A symmetrical response signal, which represents the digital information without the asymmetry, may be represented mathematically by a function h(t) whose nominal peak amplitude is one. A simplified model for the asymmetry in the asymmetric response signal g(t) is quadratic in nature, and can be expressed according to equation (2): EQU g(t)=h(t)+c.sub.asym h.sup.2 (t) (2)
According to equation (1) the coefficient c.sub.asym is half the value of the asymmetry.
In conventional data retrieval systems, the asymmetry of the response signal is compensated for by targeting different thresholds for positive and negative pulses. This is usually accomplished in a digital domain after the response signal has been sampled and digitized. This method, however, doesn't compensate for harmonic distortion that is present in the original asymmetric response signal as it appears clearly in equation (2).